1. Field Of The Invention
The invention relates to an annular gap-type ball mill for pulverizing continuously in particular mineral hard substances comprising an upright grinding container closed by a cover and housing a rotor whose cone-shaped outer surface limits with the cone-shaped inner surface of the grinding container a grinding gap communicating with a feed aperture and containing grinding pellets, the rotor having a top portion being adapted in its shape to the surface of the cover and including with its range an outlet opening.
2. Description Of The Related Art
Mineral hard substances (Mohs' hardness&gt;5) such as corundum, circonium dioxide, alumina, silicon carbide and similar substances have been pulverized predominantly hitherto by iron balls in ball mills. Considerable residence times of the grinding material in the grinding chamber are involved therewith and all of the elements contacting the grinding material and the iron balls are exposed to a very strong wear. Further, the noise developing with the grinding operation is very disturbing. Moreover, as an additional disadvantage of said ball mills, the abrasion of the iron balls gets into the grinding material and requires chemical processes to be washed out by complicated, expensive means.
Annular gap-type ball mills of the above mentioned type (German laid-open print No. 28 48 479) are supposed to incorporate an improvement over conventional ones, but they are less suited for the size reduction of mineral hard substances, and they are only economic in view of the comminution of considerably softer substances such as chalk or the like. This is particularly due to the behaviour of the grinding balls or pellets in the grinding gap.
While the grinding pellets pumped together with the grinding stock through the feed opening from below or by a hollow shaft of the rotor from above into the grinding gap, first are moved up in the grinding gap due to the pressure of the feed pump by which the grinding stock suspension is pressed into the annular gap-type mill and by the rotational movement of the rotor, they sink down by gravity with decreasing pumping pressure so that a grinding operation may not take place in the upper part of the grinding gap. This may be avoided by increasing the feed pump pressure or the flow of the grinding material such as to keep the grinding pellets in the upper portion of the grinding gap. This involves the risk for the grinding pellets to be discharged together with the grinding stock thus causing a reduction of the grinding output. Experience has shown that with an average flow rate of the grinding material, only about the lower half of the grinding gap is fully utilized for the grinding operation, while the grinding output obtainable theoretically is only half-realized. Further, the high packing density of the grinding pellets in the lower part of the grinding gap causes a high wear of the surfaces of the rotor and the grinding container. The rotor may be even blocked, after all, upon a short rest period of the rotor and the feed pump. Said risk shall be excluded with an annular gap-type ball mill of the above mentioned design, in that the lower end of the rotor is provided with an impeller which, however, will intensify only another disadvantage of the annular gap-type ball mill to the effect that the grinding pellets, which do not sink down, are increasingly pumped with the grinding material to the discharge opening to thus be lost for the grinding operation. Moreover, the impeller is exposed to a great wear caused by the grinding pellets and the grinding material. Sometimes, screens are used to retain the grinding pellets in the grinding gap; however, they will inhibit the discharging of the grinding material to even stop such a discharge if they are clogged with grinding material and grinding pellets.
A uniform flow of grinding stock through the grinding chamber shall be ensured, with the mentioned annular gap-type ball mill, by a relatively high collecting chamber above the rotor which chamber is limited by the convexly curved end face of the rotor top portion and by the respectively convexly curved inner face of the cover of the grinding container, the collecting chamber communicating directly with the outlet opening. This collecting chamber may not contribute to the object of retaining grinding pellets in the grinding gap.
The more difficult starting of the rotor and the wear markings at rotor and grinding container due to the concentration of the grinding pellets at the lower end of an annular grinding gap inclined relative to the vertical line shall be avoided, according to another annular gap-type ball mill (DE-OS No. 30 22 809) in that rotor and grinding container are drawn apart axially, in case of demand, to enlarge the grinding gap. To this effect, complicated technical measures are required which result in a more expensive equipment. However, an increased effectivity of the grinding pellets in the grinding gap, i.e. utilization of the total grinding gap height for the grinding operation are achieved but to a slight extent only. In fact, the grinding pellets present in the grinding gap directed downwardly to the outside only follow the flow of the grinding stock instead of the counteracting it such as in the upwardly directed grinding gap so that the operation effected in this part of the grinding gap is only insufficient.
Another known annular gap-type ball mill (DE-OS No. 28 11 899) comprises a grinding stock container whose inner surface confines a grinding chamber into which dips a conical cam body, the inner surface of the grinding stock container and the displacement body being of an annular double-cone design. According to a probable further embodiment, the surfaces confronted with the grinding chamber may be rough or include elevations or recesses such as ribs, grooves, pins or the like.
However, this would cause an unbearable wear just with the grinding of hard materials. Neither the annular gap-type ball mill by itself nor said specific design lend themselves to pulverizing mineral hard substances.
In contradistinction thereto, the problem underlying the invention is to improve an annular gap-type ball mill of the foregoing type so that, by an increased effectivity of the grinding pellets in the grinding gap, an economic and technically perfect pulverization, even of mineral hard substances, is possible.